In a Thermodynamic Process, the Internal Energy of a System

Multiple Choice

In a thermodynamic process, the internal energy of a system in a container with adiabatic walls decreases by 800 J. Which statement is correct?

A) The system lost 800 J by heat transfer to its surroundings.
B) The system gained 800 J by heat transfer from its surroundings.
C) The system performed 800 J of work on its surroundings.
D) The surroundings performed 800 J of work on the system.
E) The 800 J of work done by the system was equal to the 800 J of heat transferred to the system from its surroundings.